The four serotypes of Dengue virus (DENV) cause dengue fever (DF) and dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) in humans. Studies suggest that DHF/DSS is an immunopathogenic disease in which T cell responses may be dysregulated. Understanding the mechanisms that regulate the balance between T cell mediated pathology versus protection is crucial for developing therapies and, importantly, a safe dengue vaccine. By alternately passaging a DENV between mice and mosquito cells, we have naturally selected for strains that induce robust T cell responses in mice, and we have demonstrated a critical role for CD8+ T cells in limiting DENV infection in mice. Due to the broad specificity and antiviral role of the CD8+ T cell response to DENV2 in mice, we hypothesize that the murine and human CD8+ T cell response to DENV3 is broad, targeting most viral proteins, and these CD8+ T cells protect against DENV3 infection in the host. Specifically, we will examine the contribution of CD8+ T cells to protection versus pathogenesis using a mouse model of DENV3 infection (Aims 1 and 2), and we will define the specificity and role of human CD8+ T cell response in the host response to DENV3 infection using HLA transgenic mice and samples derived from people who have recovered from primary DENV3 infection (Aims 3 and 4). This project focuses on DENV3 because epidemiologic studies suggest that certain DENV serotypes, genotypes, and the sequence of DENV infections may be associated with DHF/DSS, and little is known about DENV3 infection as compared with DENV2, although DENV3 co-circulates with DENV2 in endemic countries, and DENV3 has been responsible for recent epidemics in South Asia and Latin America.